Currently, a sporty, high-performance car presents a rear-wheel drive and is provided with a self-locking differential aiming at maximising motive torque transmitted by the rear wheels to the road surface in all conditions.
Rear-wheel drive with self-locking differential is a premium for sporty driving in either optimal grip conditions (dry road) or reasonable grip conditions (wet road); however, such solution in combination with a high motive torque and large-sized tyres entails very difficult and potentially dangerous driving in poor grip conditions (flooded or icy road).
The use of permanent or insertable four-wheel drive has been proposed in order to improve the driveability of a sporty car in poor grip conditions.
Permanent four-wheel drive allows to considerably improve the behaviour of the motor vehicle in poor grip conditions, but presents the drawback of increasing loss of torque in the transmission system and conferring to the car a behaviour in optimal grip conditions which is not always appreciated by drivers.
Insertable four-wheel drive allows the motor vehicle driver to decide whether to use rear-wheel drive or four-wheel drive; in this way, the driver may use rear-wheel drive in optimal grip conditions and four-wheel drive in poor grip conditions. However, insertable four-wheel drive is complex and costly to manufacture; furthermore, the driver may not realise the presence of sheets of ice on the road and may not therefore engage four-wheel drive in a timely fashion.
Patent application IT20043000068 describes an insertable four-wheel drive motor vehicle, which is provided with an engine having a crankshaft, a pair of main driving wheels constantly connected to the crankshaft by interposition of a gearbox provided with a first clutch, and a pair of secondary driving wheels, which may be selectively connected to the crankshaft by means of an insertable transmission system; the insertable transmission system presents a second clutch, which on one end is connected with fixed transmission ratio to the crankshaft upstream of the gearbox and on the other is connected with fixed transmission ratio to the secondary driving wheels. The insertable transmission system transfers the motive torque to the front wheels when a difference between the rotation speed of the front wheels and the rotation speed of the rear wheels occurs, i.e. essentially when the rear wheels slip. Such control strategy may satisfactorily solve the problems of traction control, but has been demonstrated totally unsuitable for intervening also on the stability of the car.